1. Field of the Invention
This invention relates to the nondestructive testing of the subsurface structure of an object and, more particularly, to employing a sensing probe capable of continuously moving and operating over curved surfaces.
2. Descripton of the Prior Art
An ever-increasing number of aircraft parts are being constructed from laminated layers of high strength material because parts fabricated in this manner are lighter and easier to manufacture (especially where the part includes compound curves) than comparable metal parts. However, such laminate composite parts are susceptible to bonding flaws which cannot be detected by visual inspection. In addition, the resin used in the lamination process is vulnerable to degradation from exposure to ultraviolet rays, deleteriously affecting the structural integrity of the part and remaining virtually undetectable until resulting in a fracture. The impact of a heavy object on a laminate composite part can also cause internal cracks while leaving the surface undamaged.
The need to provide for the nondestructive subsurface inspection of laminate composite parts, both immediately after their manufacture as well as periodically in the field, has given rise to two systems which inspect the subsurface material using ultrasonic waves: through transmission and pulse echo.
Through transmission apparatus uses an emitting transducer to direct ultrasonic waves at a normal angle of incidence to the opposing surface of the part to be tested. A receiving transducer is located on the opposite side of the part and is aligned with the sending transducer. The part is inspected for flaws by measuring the attenuation suffered by the ultrasonic waves in passing through the part, and comparing the actual attenuation with the value that would be obtained in the absence of any irregularities.
There are several inherent constraints that limit the utility of testing with through transmission apparatus. The receiving transducer must be located on the side of the thickness of material being tested that lies opposite the side that faces the emitting transducer, and the receiving transducer must remain aligned with the emitting transducer. This requires access to the back side of the composite thickness being tested, and precludes using through transmission apparatus to test assembled parts which enclose a volume, for example, a wing which encloses a fuel tank. If such parts are to be tested using through transmission, the individual sides comprising the part must be tested before final assembly. Periodic field testing of such parts after their assembly is not possible.
In addition, reflection from the opposing surface attenuates the strength of the incident wave that continues through the test object, with the amount of attenuation increasing with an increase in the difference between the angle of incidence and normality. Thus, although it is not required that the incident ultrasonic wave be precisely normal, it is desirable to obtain an angle of incidence as close to normal as possible in order to maximize the amplitude of the wave passing through the test object and remain safely above the noise level of the receiving transducer.
The requirements of keeping the receiving and emitting transducers aligned and maintaining an approximately normal angle of incidence for the impinging ultrasonic waves give rise to considerable design problems when the test object includes curved surfaces. The problem of testing over curved surfaces has typically been resolved by interrupting the testing to change the position of the part. A recent development intended to reduce the time consumed by the incessant interruptions required to reposition the test part is disclosed by U.S. patent application Ser. No. 760,265: "Surface Tracking Apparatus", invented by Gerald D. Garner and Walter E. Wozniak and filed on July 29, 1985.
The apparatus of Garner et al. keeps the part being tested stationary, and instead continually orients the emitting transducer to maintain normal incidence between the ultrasonic waves and the opposing surface of the part, as well as orienting the receiving transducer to keep it aligned with the emitting transducer. The device successfully performs its intended function but, as can be seen from the perspective drawing provided by FIG. 5, it is quite large and would be difficult to move. In addition, it is primarily suited to inspecting laminate composite parts prior to the final assembly of the aircraft.
The gantry used in Garner et al. to move the emitting and receiving transducers relative to the part is also an integral part of devices which approach the problem of testing curved surfaces by changing the position of the part relative to the inspection apparatus, and thus similarly compromises their portability and utility for testing parts on assembled vehicles.
The pulse echo system employs one transducer to both emit ultrasonic waves at an angle of incidence normal to the opposing surface of the part and subsequently receive the reflected echoes. A first echo is reflected from the opposing surface, while a second is reflected from either a subsurface anomaly or the back surface of the part. The time interval between the reception of the first and second echoes is compared with the theoretical time interval that would be required if the second echo was reflected by the back surface of the part, with an actual time interval less than the theoretical value being indicative of the second echo reflecting off of a subsurface anomaly.
The pulse echo system can be modified to enable it to ascertain the nature of any irregularity as well as its presence and subsurface depth by comparing the amplitude of the first and second echoes. However, through transmission remains the preferred system for obtaining information regarding the makeup of an anomaly because of its greater tolerance for deviation from normality of the impinging wave, which translates into fewer interruptions to change the position of a test part having a curved surface.
The apparatus disclosed in Garner et al. can also be used for pulse echo testing but, as previously alluded to, lacks the portability desired for field use and is primarily suited for the testing of laminate composite parts prior to the final assembly of the aircraft. Other pulse echo apparatus reposition the part in order to obtain normality between the emitted ultrasonic waves and the part's surface, but as they typically employ a gantry resembling that pictured in FIG. 5 of Garner et al. to move the transducer relative to the part, they are similarly not adapted for use in the field or for testing after final assembly of the aircraft.
However, as there is no need to position a receiving transducer on the opposite side of the part and keep it aligned with the emitting transducer, pulse echo apparatus is more suited to the requirements of field use than through transmission apparatus, and seeral attempts have been made to fashion a portable device for testing laminate composites using this approach. An example is provided by U.S. Pat. No. 4,470,122 issued on Sept. 4, 1984, to Dennis P. Sarr.
Sarr discloses a probe which emits a nondestructive test signal and subsequently receives its echoes. The probe is manually moved across the surface of the object being tested by the operator. A light source is affixed to the probe, and its position is monitored by two scanning light sensors which are spaced apart from each other. The device can only be used over relatively flat surfaces, and the assembly containing the two scanning light sensors must be moved when the rectangular area defined by the sweep of their scan has been covered with the probe.
The inability of the apparatus disclosed in Sarr to test a section having a curved surface is a limitation which, as previously noted, has been the focus of creative effort in the prior art. This problem is especially significant because the advantage in fabricating curved parts from laminated layers in comparison to using metal is certainly compromised when such parts cannot be easily and quickly tested in the field for subsurface flaws.